Never visited Nicaragua, but have been in Liberia CR a couple of times.
The last time, the only non-spanish speking guest besides me, asked the concierge about a possible bus ride to Managua. The concierge looked sincerely worried and explicitly described all the possible dangers and hardships a visitor would face doing that trip, and also recommended flying through SJO.
Although the opposite direction you're searching for, but the problems are the same, regardless of direction.
Better safe than sorry